1. Field of the Invention
Disclosed herein are nanoparticle compositions and methods for treating plant diseases, such as citrus greening disease.
2. Relevant Technology
Plant diseases of many types threaten the world's food supply. Among the most devastating is citrus greening disease, which affects citrus trees, primarily orange trees. There is no cure for citrus greening disease and efforts to control the disease have been slow because infected citrus plants are difficult to maintain, regenerate, and study. The only known solution is to spray trees with insecticides to kill the insects responsible for spreading the disease. In June 2014, the United States Department of Agriculture allocated US $31.5 million for research to help combat citrus greening disease.
Citrus greening disease is distinguished by the common symptoms of yellowing of the veins and adjacent tissues of the citrus plant, followed by yellowing or mottling of the entire leaf, followed by premature defoliation, dieback of twigs, decay of feeder rootlets and lateral roots, and decline in vigor. This is ultimately followed by death of the entire plant. Affected trees have stunted growth, bear multiple off-season flowers (most of which fall off), and produce small, irregularly-shaped fruit with a thick, pale peel that remains green at the bottom. Fruit from these trees tastes bitter.
Citrus greening disease is caused by a vector-transmitted pathogen. The causative agents are motile bacteria, Candidatus Liberibacter spp. Transmission is by insects: the Asian citrus psyllid (Sternorrhyncha: Psyllidae), Diaphorina citri or, in Africa, by Trioza erytreae, the African citrus psyllid, also known as the 2-spotted citrus psyllid. The disease was first described in 1929 and first reported in China in 1943. The African variation was first reported in 1947 in South Africa, where it is still widespread.
The causative agents are fastidious phloem-restricted, gram-negative bacteria in the gracilicutes clade. The Asian form, L. asiaticus is heat tolerant. This means the greening symptoms can develop at temperatures of up to 35° C. The African form, L. africanum, is heat sensitive and in its case, symptoms only develop when the temperature is in the range 20-25° C. The bacteria are transmitted by the psyllid vectors and also by graft transmission. Although Trioza erytreae is the natural vector of African citrus greening and Diaphorina citri is the natural vector of Asian citrus greening, either psyllid can in fact transmit either of the greening agents under experimental conditions.
Researchers at the Agricultural Research Service have used lemon trees infected with citrus greening disease to infect periwinkle plants in an effort to study the disease. Periwinkle plants are easily infected with the disease and respond well when experimentally treated with antibiotics. Researchers are testing the effect of penicillin G sodium and biocide 2,2-dibromo-3-nitrilopropionamide as potential treatments for infected citrus plants based on preliminary results observed when applied to infected periwinkle.
Distribution of citrus greening disease is primarily in tropical and subtropical Asia. It has been reported in all citrus-growing regions in Asia except Japan. The disease has affected crops in China, Taiwan, India, Sri Lanka, Malaysia, Indonesia, Myanmar, Sri Lanka, the Philippines, Pakistan, Thailand, the Ryukyu Islands, Nepal, Saudi Arabia, and Afghanistan. Areas outside Asia have also reported the disease: Réunion, Mauritius, Brazil, and Florida (since 1998), and in several municipalities in Mexico since 2009. On Mar. 30, 2012, citrus greening disease was confirmed in a single citrus tree in Hacienda Heights, Calif. Prospects are dim for the ubiquitous backyard citrus orchards of California as residential growers are unlikely to consistently use the pesticides which provide effective control in commercial orchards.
Since the disease's detection in Florida City and Homestead, 90,000 acres of citrus groves have been wiped out. The high cost of spraying to kill off some of the psyllids is pushing some growers to the financial brink. The average cost of producing an acre of oranges is $1,800, nearly double what it cost in 1995. A 2012 analysis estimated the disease has cost growers $4.6 billion and resulted in the loss of about 8,000 jobs.
In the heyday of Florida citrus, around 1970, the number of acres with orange, grapefruit, and specialty fruit orchards surpassed 900,000. Today, it is reportedly slightly more than 500,000 acres.
Accordingly, there has been a long-felt but unsatisfied need to find a cure for citrus greening disease. The need to find a cure for citrus greening disease has become particularly acute as large numbers of citrus trees in the United States have been devastated. Notwithstanding many attempts to control the disease none have been particularly effective and none actually provide a reliable cure.